NDARC Technical Report No. 3 (1989)
ABSTRACT
A study was undertaken to examine the effects of methadone, as used in the methadone maintenance program, on human performance skills which are related to those required to drive a motor vehicle with safety.
The tests used for the study were chosen for their relevance to driving as well as for the distinctive properties of the opioids.
Three groups of volunteers from the methadone programs in Sydney were chosen to represent various stages of progress by clients within the program. These were (i) those beginning on the program; (ii) those receiving an increase in dosage of 10 mg methadone; and (iii) those who have been stabilized on a dose of
methadone for a period of at least six months.
The interaction between methadone and two other drugs commonly used by clients on a methadone program were also examined. These were (i) alcohol, to produce a mean blood alcohol concentration at peak of 0.064 g per 100 ml blood; and (ii) a therapeutic dose of the benzodiazepine, diazepam (15 mg).
The mean dose of methadone taken by all of the clients within the study was 70 mg (range 15 to 150 mg). The mean dose for the individual groups was (i) stabilised group, 85 mg (range 40 to 150 mg); (ii) increased dose group, 67 mg (range 40 to 135 mg); and (iii) those beginning the program, 38 mg (range 15 to 60 mg).
Two control groups were employed: a group of ex-users of heroin who were drug-free and a group of non-opioid users.
Volunteers for the control groups and the stabilised methadone group attended the laboratory on four occasions (days). On days 1 and 2, all subjects after practice on the tests, completed the test battery twice, before and after the methadone clients had received their daily dose of methadone. An interval of one hour was allowed after the methadone dose to allow time for absorption. On day 3 all clients from the control groups and the stabilised methadone groups received alcohol and on day 4, diazepam.
The test battery proved to be sensitive to the effects of alcohol and diazepam at the doses used.
There was no evidence for an effect of the acute dose of methadone on any of the experimental groups of clients on the methadone program. The insensitivity of these tests of skill performance to the acute effect of methadone on the clients within the methadone maintenance program indicates that these clients should not be considered as impaired in their ability to perform complex tasks such as driving a motor vehicle.
Both alcohol and diazepam produced a significant decrement in the performance on the test battery by the control groups and the stabilised methadone clients. However, there was no difference in the intensity of this effect between the groups. There was no evidence for an interaction between methadone and either alcohol or diazepam in the group of methadone clients stabilised on the program.
The overall scores on the test battery showed a trend to poorer performance by the methadone clients. This difference achieved significance for the stabilised group of methadone clients. However, the differences in overall performance between the methadone groups and the controls were considerably smaller than those produced by the acute doses of alcohol or diazepam.
These differences in overall performance were not attributable to the acute dose of methadone. The possibility that they are related to a chronic effect of methadone was examined. There was a correlation between the methadone dose and the overall performance measure, but this accounted for only 8% of the variance and was not the most important variable associated with this effect.
There was no difference in the performance of those stabilised methadone clients who received less than 80 mg methadone, from those who received 80 mg or more per day. The dose of 80 mg methadone is that considered in the National Methadone Guidelines (1988) as being the threshold dose for what is described as a "high dose".
It is considered that the differences in overall performance between the methadone clients and the controls can be interpreted in a manner which does not involve the pharmacological effects of methadone. It is suggested that factors including unemployment, life-style, social and personality disorders could play a
contributory role.